WeakRef(3) User Contributed Perl Documentation WeakRef(3)NAME
WeakRef -- an API to the Perl weak references
SYNOPSIS
use WeakRef;
for($i=0; $i<100000; $i++) {
my $x = {};
my $y = {};
$x->{Y} = $y;
$y->{X} = $y;
weaken($x->{Y});
} # no memory leak
if(isweak($ref)) {
}
DESCRIPTION
A patch to Perl 5.005_55 by the author implements a core API for weak references. This module is a Perl-level interface to that API,
allowing weak references to be created in Perl.
A weak reference is just like an ordinary Perl reference except that it isn't included in the reference count of the thing referred to.
This means that once all references to a particular piece of data are weak, the piece of data is freed and all the weak references are set
to undef. This is particularly useful for implementing circular data structures without memory leaks or caches of objects.
The command
use WeakRef;
exports two symbols to the user's namespace by default: "weaken" and "isweak". "weaken" takes a single argument, the reference to be
weakened, and returns the same value. The idiom
weaken($this->{Thing}->{Parent} = $this);
is useful.
The "isweak" command takes a single parameter and returns true if the parameter is a weak reference, undef otherwise.
BUGS
None known.
AUTHOR
Tuomas J. Lukka lukka@iki.fi
Copyright (c) 1999 Tuomas J. Lukka. All rights reserved. This program is free software; you can redistribute it and/or modify it under the
same terms as perl itself.
BLATANT PLUG
This module and the patch to the core Perl were written in connection with the APress book `Tuomas J. Lukka's Definitive Guide to Object-
Oriented Programming in Perl', to avoid explaining why certain things would have to be done in cumbersome ways.
perl v5.12.1 1999-02-23 WeakRef(3)
Check Out this Related Man Page
Scalar::Util(3pm) Perl Programmers Reference Guide Scalar::Util(3pm)NAME
Scalar::Util - A selection of general-utility scalar subroutines
SYNOPSIS
use Scalar::Util qw(blessed dualvar isweak readonly refaddr reftype tainted
weaken isvstring looks_like_number set_prototype);
# and other useful utils appearing below
DESCRIPTION
"Scalar::Util" contains a selection of subroutines that people have expressed would be nice to have in the perl core, but the usage would
not really be high enough to warrant the use of a keyword, and the size so small such that being individual extensions would be wasteful.
By default "Scalar::Util" does not export any subroutines. The subroutines defined are
blessed EXPR
If EXPR evaluates to a blessed reference the name of the package that it is blessed into is returned. Otherwise "undef" is returned.
$scalar = "foo";
$class = blessed $scalar; # undef
$ref = [];
$class = blessed $ref; # undef
$obj = bless [], "Foo";
$class = blessed $obj; # "Foo"
dualvar NUM, STRING
Returns a scalar that has the value NUM in a numeric context and the value STRING in a string context.
$foo = dualvar 10, "Hello";
$num = $foo + 2; # 12
$str = $foo . " world"; # Hello world
isvstring EXPR
If EXPR is a scalar which was coded as a vstring the result is true.
$vs = v49.46.48;
$fmt = isvstring($vs) ? "%vd" : "%s"; #true
printf($fmt,$vs);
isweak EXPR
If EXPR is a scalar which is a weak reference the result is true.
$ref = $foo;
$weak = isweak($ref); # false
weaken($ref);
$weak = isweak($ref); # true
NOTE: Copying a weak reference creates a normal, strong, reference.
$copy = $ref;
$weak = isweak($copy); # false
looks_like_number EXPR
Returns true if perl thinks EXPR is a number. See "looks_like_number" in perlapi.
openhandle FH
Returns FH if FH may be used as a filehandle and is open, or FH is a tied handle. Otherwise "undef" is returned.
$fh = openhandle(*STDIN); # *STDIN
$fh = openhandle(*STDIN); # *STDIN
$fh = openhandle(*NOTOPEN); # undef
$fh = openhandle("scalar"); # undef
readonly SCALAR
Returns true if SCALAR is readonly.
sub foo { readonly($_[0]) }
$readonly = foo($bar); # false
$readonly = foo(0); # true
refaddr EXPR
If EXPR evaluates to a reference the internal memory address of the referenced value is returned. Otherwise "undef" is returned.
$addr = refaddr "string"; # undef
$addr = refaddr $var; # eg 12345678
$addr = refaddr []; # eg 23456784
$obj = bless {}, "Foo";
$addr = refaddr $obj; # eg 88123488
reftype EXPR
If EXPR evaluates to a reference the type of the variable referenced is returned. Otherwise "undef" is returned.
$type = reftype "string"; # undef
$type = reftype $var; # SCALAR
$type = reftype []; # ARRAY
$obj = bless {}, "Foo";
$type = reftype $obj; # HASH
set_prototype CODEREF, PROTOTYPE
Sets the prototype of the given function, or deletes it if PROTOTYPE is undef. Returns the CODEREF.
set_prototype &foo, '$$';
tainted EXPR
Return true if the result of EXPR is tainted
$taint = tainted("constant"); # false
$taint = tainted($ENV{PWD}); # true if running under -T
weaken REF
REF will be turned into a weak reference. This means that it will not hold a reference count on the object it references. Also when the
reference count on that object reaches zero, REF will be set to undef.
This is useful for keeping copies of references , but you don't want to prevent the object being DESTROY-ed at its usual time.
{
my $var;
$ref = $var;
weaken($ref); # Make $ref a weak reference
}
# $ref is now undef
Note that if you take a copy of a scalar with a weakened reference, the copy will be a strong reference.
my $var;
my $foo = $var;
weaken($foo); # Make $foo a weak reference
my $bar = $foo; # $bar is now a strong reference
This may be less obvious in other situations, such as "grep()", for instance when grepping through a list of weakened references to
objects that may have been destroyed already:
@object = grep { defined } @object;
This will indeed remove all references to destroyed objects, but the remaining references to objects will be strong, causing the
remaining objects to never be destroyed because there is now always a strong reference to them in the @object array.
DIAGNOSTICS
Module use may give one of the following errors during import.
Weak references are not implemented in the version of perl
The version of perl that you are using does not implement weak references, to use "isweak" or "weaken" you will need to use a newer
release of perl.
Vstrings are not implemented in the version of perl
The version of perl that you are using does not implement Vstrings, to use "isvstring" you will need to use a newer release of perl.
"NAME" is only available with the XS version of Scalar::Util
"Scalar::Util" contains both perl and C implementations of many of its functions so that those without access to a C compiler may still
use it. However some of the functions are only available when a C compiler was available to compile the XS version of the extension.
At present that list is: weaken, isweak, dualvar, isvstring, set_prototype
KNOWN BUGS
There is a bug in perl5.6.0 with UV's that are >= 1<<31. This will show up as tests 8 and 9 of dualvar.t failing
SEE ALSO
List::Util
COPYRIGHT
Copyright (c) 1997-2007 Graham Barr <gbarr@pobox.com>. All rights reserved. This program is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.
Except weaken and isweak which are
Copyright (c) 1999 Tuomas J. Lukka <lukka@iki.fi>. All rights reserved. This program is free software; you can redistribute it and/or
modify it under the same terms as perl itself.
perl v5.12.1 2010-05-13 Scalar::Util(3pm)